Plastic in the Oceans May Help Some Species Proliferate Like Weeds

Plastic's durability helped to make it a popular miracle material in the early 20th century. Its omnipresence, however, may now be disrupting ecosystems in some surprising ways. A new study by researchers at the Scripps Institution of Oceanography in La Jolla, Calif., shows that the concentration of plastic has increased by 100 times over the past 40 years in the North Pacific Subtropical Gyre—an enormous calm spot in the middle of a clockwise rotation of ocean currents that falls between East Asia and the West Coast of the U.S., with Hawaii as its approximate midpoint. The size of the area is estimated to be more than 18 million square kilometers.

The study, published online on May 9 in Biology Letters, also documented for the first time a rise in egg densities of Halobates sericeus, a water strider that lays its eggs on floating objects. The team collected and analyzed data on bits of plastic less than five millimeters across in the North Pacific Ocean, including records from two recent voyages, published data from other sources and data developed from archived samples in the Scripps collection taken in the early 1970s. Author Miriam Goldstein, who is a biological oceanography Ph.D. candidate at Scripps, notes that a 2011 study that examined the North Atlantic Subtropical Gyre found no increase in plastic since 1986.

Higher concentrations of floating plastic debris offer more opportunities for the pelagic strider to lay eggs. This marine insect—closely related to pond striders—spends its entire life out on the open ocean and takes its place in the food web by consuming zooplankton and larval fish and being eaten by crabs, fish and seabirds.

Floating objects are historically rare in the North Pacific. “Striders would have been lucky to find a feather or a bit of floating wood,” Goldstein says. Now floating plastic pieces are more common and offer a surface on which striders can lay their bright yellow, rice grain–size eggs.

Although researchers found an increase in eggs, they did not find an increase in the insects themselves. That could be because there were not enough samples from the early 1970s with which to adequately compare them, but equally likely crabs or small surface-feeding fish may be eating the eggs, Goldstein notes.

Researchers are concerned that this proliferation of plastic may be giving striders, microbes, animals and plants that grow directly on the plastic an advantage over oceanic animals that are not associated with hard surfaces, such as fish, squid, tiny crustaceans and jellyfish. “While these organisms [that grow directly on the plastic] are native, they're kind of like weeds,” Goldstein explains, in that they grow, reproduce and die quickly. In contrast, the organisms in the water column tend to be more biodiverse. More than half of the ocean is part of the subtropical gyres, and changing the way that these gyres function by adding lots of plastic trash could have unpredictable consequences. “While our study only looks at one little insect in one area of the ocean, it shows that tiny pieces of plastic do have the potential to alter the ecology of the open sea,” she says.